The mechanism for the contextual hint, with tier-2 fallback; the
tier-3 corpus lands in later phases.
- new CommandNode `hint_id` field (all None for now)
- AppCommand::Hint + HINT grammar node + REGISTRY + dispatch
- F1 read-only overlay in handle_key (buffer/cursor/memo untouched)
- note_hint* renderers; hint_id_for_input_in_mode (shared selection
helper refactored out of usage_keys_for_input_in_mode)
- last_error_hint_key + friendly::error_hint_class classifier
- catalogue: help.app.hint / parse.usage.hint / hint.getting_started
- +12 tests; 2483 pass / 1 ignored, clippy clean
claude@clouddev1
050b36391e